Characterization of immune alterations induced by a conditioned aversive stimulus

In this study, we investigated the immune alterations induced in rats by an aversive conditioned stimulus that had been developed through pairings with electric shock. The results showed that the conditioned stimulus induced a pronounced suppression of the mitogenic responsiveness of splenic and blood lymphocytes and a reduction in splenic natural killer cell activity. In contrast, the conditioned stimulus did not induce any alteration in the mitogenic responsiveness of lymphocytes from the mesenteric lymph nodes. The reduction in the mitogenic responsiveness of splenic lymphocytes was not related to a reduction in the level of interleukin-2 (IL-2) production, as splenic lymphocytes from subjects exposed to the conditioned stimulus showed normal levels of IL-2. Plasma corticosterone measurements showed that glucocorticoid secretion was related to the alteration of the mitogenic responsiveness of blood lymphocytes. However, plasma corticosterone levels were not related to any of the other immune measures. These findings establish that a signal for an aversive event can have a pronounced effect on immune function, but that such an effect is dependent on the particular compartment of the immune system studied. These results support the claim that glucocorticoids can induce immune alterations, but they suggest that additional pathways must be involved in the immune alterations induced by a conditioned aversive stimulus.